It is generally accepted that increasing oil prices and national legislation requiring the reduction of automotive carbon dioxide emissions force tire and rubber producers to contribute to produce “fuel-efficient” and thus fuel- or gas-saving tires. One general approach to obtain fuel-efficient tires is to produce tire formulations which have reduced hysteresis loss. A major source of hysteresis in vulcanized elastomeric polymers is attributed to free polymer chain ends, i.e. the section of the elastomeric polymer chain between the last cross-link and the end of the polymer chain. This free end of the polymer does not participate in the efficient elastically recoverable process and, as a result, energy transmitted to this section of the polymer is lost. The dissipated energy leads to a pronounced hysteresis under dynamic deformation. Another source of hysteresis in vulcanized elastomeric polymers is attributed to an insufficient distribution of filler particles in the vulcanized elastomeric polymer composition. The hysteresis loss of a cross-linked elastomeric polymer composition is related to its tan δ value at 60° C. (see ISO 4664-1:2005; Rubber, Vulcanized or thermoplastic; Determination of dynamic properties—part 1: General guidance). In general, vulcanized elastomeric polymer compositions having relatively small tan δ values at 60° C. are preferred as having lower hysteresis loss. In the final tire product, this translates into a lower rolling resistance and better fuel economy.
It is generally accepted that a lower rolling resistance tire can be made at the expense of deteriorated wet grip properties. For example, if, in a random solution styrene-butadiene rubber (random SSBR), the polystyrene unit concentration is relatively reduced with respect to the total polybutadiene unit concentration, and the 1,2-polydiene unit concentration is kept constant, the SSBR glass transition temperature is reduced and, as consequence, both tan δ at 60° C. and tan δ at 0° C. are reduced, generally corresponding to improved rolling resistance and deteriorated wet grip performance of the tire. Similarly, if, in a random solution styrene-butadiene rubber (random SSBR), the 1,2-polybutadiene unit concentration is relatively reduced with respect to the total polybutadiene unit concentration, and the polystyrene unit concentration is kept constant, the SSBR glass transition temperature is reduced and, as consequence, both tan δ at 60° C. and tan δ at 0° C. are reduced, generally corresponding to improved rolling resistance and deteriorated wet grip performance of the tire. Accordingly, when assessing the rubber vulcanizate performance correctly, both the rolling resistance, related to tan δ at 60° C., and the wet grip, related to tan δ at 0° C., should be monitored along with the tire heat build up.
One generally accepted approach to reducing hysteresis loss is to reduce the number of free chain ends of elastomeric polymers. Various techniques have been described in the literature, including the use of “coupling agents,” such as tin tetrachloride, which may functionalize the polymer chain end and react with components of an elastomeric composition, for example with a filler or with unsaturated portions of a polymer. Examples of such techniques and coupling agents are described in the following patents: U.S. Pat. Nos. 3,281,383 3,244,664 and 3,692,874 (for example, tetrachlorosilane); U.S. Pat. Nos. 3,978,103; 4,048,206; 4,474,908; 6,777,569 (blocked mercaptosilanes) and U.S. Pat. No. 3,078,254 (a multi-halogen-substituted hydrocarbon, such as 1,3,5-tri(bromo methyl)benzene); U.S. Pat. No. 4,616,069 (tin compound and organic amino or amine compound); and U.S. 2005/0124740.
The use of “coupling agents” as reactants with living polymers more often than not leads to the formation of polymer blends comprising one fraction of linear or uncoupled polymers and one or more fractions comprising more than two polymer arms at the coupling point. The reference article “Synthesis of end-functionalized polymer by means of living anionic polymerization,” Journal of Macromolecular Chemistry and Physics, 197, (1996), 3135-3148, describes the synthesis of “polystyrene-containing” and “polyisoprene-containing” living polymers with hydroxy (—OH) and mercapto (—SH) functional end caps, obtained by reaction of the living polymers with haloalkanes containing silyl ether and silyl thioether functions. The tertiary-butyldimethylsilyl (TBDMS) group is preferred as a protecting group for the —OH and —SH functions in the termination reactions, because the corresponding silyl ethers and thioethers are found to be both stable and compatible with anionic living polymers.
WO2007/047943 describes the use of a silane-sulfide omega chain end modifier to produce a chain end-modified elastomeric polymer, used as component in a vulcanized elastomeric polymer composition or a tire tread. Although cured rubber hysteresis properties can be improved significantly, the effect is limited since only one polymer chain end can be functionalized by using the modifier compound described.
WO9706192 describes a copolymer produced by copolymerizing an alkenyl-substituted aromatic hydrocarbon and a conjugated diene with a protected functional organometallic initiator. WO9706192 does not provide cured polymer or copolymers, so that the impact of the moiety derived from the organometallic initiator in the (co)polymer on cured rubber hysteresis properties is not known.
WO2011031943 describes a polymerization initiator comprising at least two protected primary amine groups and at least one alkali or alkaline earth metal, as well as the polymers made by using the specified polymerization initiators.
WO2011082277 reports on a “metallated aminosilane compound for initiating an anionic polymerization” comprising the reaction product of (1) at least one metallating agent and (2) at least one specific alkylaminosilane compound. While the components (1) and (2) are described in detail, hardly any structural information is given with respect to the “metallated aminosilane compound for initiating an anionic polymerization”, and none is given for the polymer structure element derived from the initiator system used.
There is a need for modification methods and resulting modified macromolecular compounds that can be used to further optimize dynamic silica and carbon black vulcanizate properties, including low hysteresis properties, corresponding to a high wet grip and low rolling resistance property in tires. In addition, there is a need to further decrease the vulcanizate heat built up during thermal exposure and under mechanical stress. These needs have been met by the following invention. Particularly, there is a need for an efficient organometal initiator which improves cured modified polymer hysteresis properties when used either alone or in combination with chain end modifiers, the combined embodiment leading to significantly enhanced performance property improvements.